Thermal process for in-situ decomposition of oil shale



2,780,449 THERMAL PROCESS FOR IN-SITU DECOMPOSITION OF OIL SHALE 1957 F. R. FISHER ETAL Filed Dec. 26, 1952 mmDhODmhm M435 :0

, FRANK R. FISHER HARRY L. PELZER ATTORNEYS United States Patent THERMAL PROCESS FOR IN-SITU DECOMPOSI- TION OF OIL SHALE Frank R. Fisher, Hastings on Hudson, N. Y., and Harry L. Pelzer, Catoosa, Okla, assignors to Sinclair Oil and Gas Company, Tulsa, Okla, a corporation of Maine Application December 26, 1952, Serial No. 328,144

5 Claims. (Cl. 262-3) Our invention relates to improvements in the recovery of oil from subsurface oil shale formations and particularly relates to a process providing in situ means for decomposition of the oil shale and for recovery of its oil content. Although vast reserves of oil in the form of shale oil are potentially available both in this country and abroad, the high costs of mining the oil shale, crushing and grinding it as a preliminary to any feasible recovery method such as by high temperature retorting make shale oil as a source of additional oil completely infeasible from the present economic standpoint. Thus before shale oil is available in crude form comparable to ordinary crude petroleum, the operations of stripping or drilling the overburden, mining the rock, transporting the rock to the surface and/or a suitable processing point, crushing the shale and grinding the crushed rock to a particle size permitting efiective heat treating and retorting the shale particles at high temperature must be performed. Even though potential recovery of shale oil may approximate as high as 50 to 75 gallons per ton of shale processed, the aggregate mining and processing costs impose such an economic burden that it would seem that shale oil could not become a significant energy supply factor unless a real and substantial shortage of petroleum developed.

The object of our invention is to provide a practical process for recovering shale oil by in situ means which will eliminate the costly operations of mining, transporting and processing the rock. More particularly, we accomplish in situ decomposition of the oil shale and induce flow of the shale oil in a manner permitting practical recovery by thermal means. Although application of underground burning or thermal methods has been proposed for recovery of shale oil, these proposals have failed to take into account the extreme impernieability of oil shale and hence have been unable to effect recovery except from the immediate locality of the input source of thermal energy. Obviously, a requirement that a great number of closely spaced wells must be drilled to effect recovery renders the recovery process as non-competitive with ordinary petroleum production as the requirements for mining and subsequent processing of the rock.

According to our invention, more fully described and illustrated by the accompanying drawing a plurality of wells, including an input well 3 and one or more output wells 20, may be employed in a manner similar to the well spacing patterns employed in recovery of crude oil by secondary methods. Thus an input and an output well may be drilled in a manner providing access to the oil shale formation over substantially its entire cross-section. Where outcropping of the shale formation occurs, only an input well need be drilled, and the oil may be produced from the outcrop instead of a drilled output well. A fluid pressuring medium, e. g. gas or water, is introduced through conduit 9 into the formation at an elevated pressure sufficient to penetrate the naturally occurring seams and small cracks or fissures in the shale and cause parting and cracking of the formation 15. The fluid pressuring medium can be introduced through one well until communication by pressure parting of the naturally occurring seams and cracks is established with a second well. Alternatively, the fluid pressuring medium can be introduced simultaneously through more than one well until the communication between wells has been nearly established whereupon the output well is vented and the operation is completed by fluid pressuring from the input well. Once communication between a plurality of wells, or between an input well and an outcrop, has been accomplished so as to permit passage of gases with pressure drops within the range of operation with practical compression equipment, a hot zone 12 is established within the formation at the bottom of an input well 3 by burning fuel gas or other extraneous fuel introduced via conduit 6 with air or oxygen injected through conduit 9 for a period of time sufiicient to heat a portion of the formation surrounding the input bore to a temperature promoting combustion of the carbonaceous matter left in the shale by vaporization or decomposition of the shale oils. The flow of hot combustion gases through the channels and communicating seams established during the pressure parting operation releases the trapped shale oil from the shale particles by vaporization and desorption so that the released shale oil can flow into the system of communicating passages. As the flowing oil passes into the cold unheated portions of the formation 15, condensation and increase in viscosity of course occurs, but the pressure drive effect of the combustion gases continues flow into the region of the output well which can be produced by pumping or other means.

In operation according to our invention, the potential producing field 15 is advantageously handled as a whole by drilling a plurality of wells 3 spaced in a manner designed to cover the subsurface formation efficiently but economically. Advantageously, a 5-spot or 9-spot pattern with a central input well may be employed as in conventional secondary petroleum recovery operations. The input well 3, or wells if more than one is employed, should be cased and capped for the pressure parting operation by cementing off the bore below the overburden line. As noted above, more than one well is advantageously employed in pressure parting operations until communication between the wells is nearly completed when the output Well or wells are vented. The pressure required to separate or pressure part the oil shale formation varies with the depth of the formation, its structure and permeability and the nature of the surrounding geologic structures. The pressure is best determined by actual trial in each individual formation, but as a rule of thumb, approximately one p. s. i. is required for each foot of depth. With shallow formations a pressure from about 50 percent in excess of the calculated overburden pressure to 350-400 pounds indicated as pressure above the overburden pressure may be required. If the structure or thickness of the oil shale formation requires, well bore packers may be installed at varying distances to reduce the equipment requirements for obtaining the necessary pressures for parting and cracking formation.

After formation parting and cracking has been accomplished a hot zone 12 should be established within the formation in the region of the input well or wells. The heating operation may be effected for example by piping air and fuel gas separately through conduits 9 and 6, respectively, to the bottom of the well in an explosive ratio, mixing the air and fuel at the bottom of the well and igniting the mixture by means of a pilot burner, a spark ignition device, a time delay bomb or the like. The heating operation is continued until a suflicient block of heat is built up within the formation to insure re-igni-tion of residual combustible material or an air-fuel gas mixture without resorting to ignition aids. In order to maintain the thermal drive phase of the recovery operation eficiently, the tempera-ture level in the hot zone 12 should be at least about 1,000 F. or higher in its peak region. The amount of heat required to heat up a designated portion of the formation from the input well to a given temperature is susceptible of calculation by taking into account the dimensions of the formation, the thermal properties of the oil shale and the flow rate of the combustion mixture. Usually the direct firing phase of the thermal operation requires a substantial period of time, e, g., about a month. The gas flow rate should be as high as is practicable at a pressure sufficient to maintain passage through the formation from the input well to the output wells or output zone. For example, a flow rate of about 50,000 cubic feet per hour may be employed. Assuming an oil content of about 1200 barrels per acre-foot, about 10,000 cubic feet of air per barrel of oil produced may be taken as a basis for planning the operation.

When the heat zone 12 has been established in the region of the input well 3, the operation is conducted by discontinuing burning at the bottom of the input well 3 and moving the heat zone 12 as a thermal front 14 or wave radially outward into the formation 15 in the direction of the output wells. Thus, introduction of an unheated substantially non-ccmbustible oxygen-containing gas stream is substituted so that the region of peak temperature is moved gradually outward into the formation 15 as the cold input gas flow continues at high rate from conduit 9. ln this manner, the injected gas stream acts as an effective heat transfer medium, absorbing heat as it approaches the peak temperature region and transferring heat absorbed in the process to the cooler regions of the formation 15 beyond the hot zone. Thus the hot zone is moved as a front or thermal wave 14 from the input well region through the formation toward the output Wells or outcrops. During the operation or as part of a coordinated cycle, the temperature level is maintained in the frontal zone by controlling the oxygen content of the input gas with Water, recycle field gas or other substantially inert diluents.

In order to accomplish continuous forward movement of the heat zone 14 as a wave of relatively narrow profile and high peait temperature, it is important to move the hot zone 14 out into the formation by use of a relatively cool gas drive and to avoid burn-bacl in the incoming gas drive stream. Accordingly, the operation may be conducted by cutting back the hydrocarbon content of the drive gas stream to a proportion below the explosive limit so that the mixture cannot ignite until it is well out into the formation where unburned carbonaceous matter is available to enrich the fuel-oxygen ratio to within combustible limits and where a high enough temperature for spontaneous combustion exists. For example, a gas drive at 50,000 cubic feet per hou may be employed providing a hydrocarbon content equivalent to 40 B. t. it. per cubic foot; e. g. about 4 percent methane in air, without danger of burn-back. Alternatively, alternate cycles in which an inert gas mixture; e, g. recycled field gas substantially free of oxygen, is introduced through conduit 9 as the cold gas drive until the temperature level in the hot zone begins to drop to a point endangering spontaneous reignition whereupon air or an oxygen-containing gas mixture which may be supplemented with fuel gas is substituted as the injection medium until the desired temperature level is restored in the hot zone. In this way, the hot zone 14 is moved out into the formation as a wave leaving bands of carbonaceous residue during the periods of inert gas drive. The duration of the inert gas drive cycle may approximate, for example, from several days to several weeks operation while the combustion drive cycle is usually substantially shorter in duration.

The residual carbon content of the shale after thermal displacement of the bulk of the oil content is available as fuel and advantageously is utilized as such at least in part in the combustion cycle by introduction of sufficient oxygen in the cycle gas. If the residual carbon content is low and extraneous fuel gas forms the chief combustible component, it is desirable to restrict the oxygen con tent of the oxygen-containing gas drive to less than about 6 percent in order to insure that burn-back from the hot front to the input well will not result with corresponding immobilization of the heat wave and excessive consumption of fuel.

The progress of the thermal operations within the formation can be followed by a number of means. Most directly, temperature recording wells can be drilled into the formation at appropriate intervals, or temperature rccording means can be injected laterally into the formation from the input well by horizontal drilling techniques. Indirect means, however, rovide a relatively sensitive means of following the operations involved even though allowance must be made for an inevitable time lag caused by the time required by the gas stream to traverse the formation. During combustion periods when oxygen is injected as a part of the gas drive, the proportions of oxygen, carbon dioxide and carbon monoxide in the edit:- ent gases are indicative of the relative completeness of combustion reactions and the correctness of the ratios of oxygen to the fuel in the injection gas stream.

A particular advantage of our combined method of imparting permeability to the subsurface shale oil formation by preliminary pressure parting followed by in situ decomposition of the shale to release oil by thermal means is that the essential initial permeability is established without necessity of supplying large amounts of heat or of employing costly mechanical methods. A further advange is that the thermal gas drive system, once initiated, tends to increase greatly the porosity of the shale by cracking and disintegrating the rock making it more and more permeable to subsurface gas llow, so that the efficiency of the overall gas drive operation increases as the operation proceeds. Since the region to be heated is confined by control of the gas drive operation to a peak frontal area or wave out in the region of decomposition, vaporization and oil flow, there is no need to attempt to heat the entire underground formation and the fuel costs per barrel of oil produced are small.

We claim:

1. A process for the recovery of shale oil from a subsurface oil shale formation by in situ decomposition which comprises drilling an input well to provide access to the oil shale formation, introducing a fluid pressuring medium into the formation at an elevated pressure suflicient to cause parting and cracking of the formation, establishing a hot zone within the formation by burning fuel gas at the bottom of the input well for a period of time suflicient to heat the surrounding formation to an elevated temperature promoting spontaneous combustion Within the formation; discontinuing burning at the bottom of the well, introducing an unheated substantially non-combustible oxygen-containing gas into said input well and passing said gas through said hot zone to absorb heat and to transfer absorbed heat to cooler regions of the shale formation beyond said hot zone, thereby mov ing said hot zone gradually outward into the formation; introducing into said hot zone a cool oxygen-containing combustion-supporting gas to promote forward movement of said hot zone as a thermal wave of relatively narrow profile and high peak temperature promoting combustion in a limited region of the formation, whereby oil is distilled in the formation in front of the advancing hot zone, and recovering oil from an output zone.

2. A process for the recovery of shale oil from a subsurface oil shale formation by in-situ decomposition which comprises drilling a plurality of input wells to provide access to the oil shale formation, introducing a fluid pressuring medium into the formation by means of said wells at an elevated pressure sufiicient to cause parting and cracking of the formation and until permeability permitting gas flow between said wells has been established, establishing a hot zone within the formation by burning fuel gas at the bottom of the input well for a period of time sufi'icient to heat the surrounding formation to an elevated temperature promoting spontaneous combustion Within the formation; discontinuing burning at the bottom of the well, introducing an unheated substantially non-combustible oxygen-containing gas into the input welt and passing said gas through said hot zone to absorb heat and to transfer absorbed heat to cooler regions of the shale formation beyond said hot zone, thereby moving said hot zone gradua'ily outward into the formation; introducing intosaid hot zone a cool oxygencontaining combustion-supporting gas to promote forward movement of said hot zone as a thermal wave of relatively narrow profile and high peak temperature promoting combustion in a limited region of the formation, whereby oil is distilled in the formation in front of the advancing hot zone, and recovering oil from an output zone.

3. The process of claim 1 wherein said combustionsupporting gas contains hydrocarbon gas in a proportion below the explosive limit whereby said gas ignites only in the presence of unburned carbonaceous matter hav- References Cited in the file of this patent UNITED STATES PATENTS l,422,204 Hoover et al. July 11, 1922 2,497,868 Dalin Feb. 21, 1950 2,584,605 Merriam et al. Feb. 5, 1952 2,584,606 Merriam et al. Feb. 5, 1952 2,596,843 Farris May 13, 1952 2,630,307 Martin Mar. 3, 1953 2,642,943 Smith et al. June 23, 1953 2,695,163 Pearce et al Nov. 23, 1954 

1. A PROCESS FOR THE RECOVERY OF SHALE OIL FROM A SUBOSURFACE OIL SHALE FORMATION BY IN-SITU DECOMPOSITION WHICH COMPRISES DRILLING AN INPUT WELL TO PROVIDE ACCESS TO THE OIL SHALE FORMATION, INTRODUCING A FLUID PRESSURING MEDIUM INTO THE FORMATION AT AN ELEVATED PRESSURE SUFFICIENT TO CAUSE PARTING AND CRACKING OF THE FORMATION, ESTABLISHING A HOT ZONE WITHIN THE FORMATION BY BURNING FUEL GAS AT THE BOTTOM OF THE INPUT WELL FOR A PERIOD OF TIME SUFFICIENT TO HEAT THE SURROUNDING FORMATION TO AN ELEVATED TEMPERATURE PROMOTING SPONTANEOUS COMBUSTION WITHIN THE FORMATION; DISCONTINUING BURNING AT THE BOTTOM OF THE WELL, INTRODUCING AN UNHEATED SUBSTANTIALLY NON-COMBUSTIBLE OXYGEN-CONTAINING GAS INTO SAID INPUT WELL AND PASSING SAID GAS THROUGH SAID HOT ZONE TO ABSORB HEAT AND TO TRANSFER ABSORBED HEAT TO COOLER REGIONS OF THE SHALE FORMATION BEYOND SAID HOT ZONE, THEREBY MOVIGN SAID HOT ZONE GRADUALLY OUTWARD INTO THE FORMATION; INTRODUCING INTO SAID HOT ZONE A COOL OXYGEN-CONTAINING COMBUSTION-SUPPORTING GAS TO PROMOTE FORWARD MOVEMENT OF SAID HOT ZONE AS A THERMAL WAVE OF RELATIVELY NARROW PROFILE AND HIGH PEAK TEMPRATURE PROMOTING COMBUSTION IN A LIMITED REGION OF THE FORMATION, WHEREBY OIL IS DISTILLED IN THE FORMATION IN FROM OF THE ADVANCING HOT ZONE, AND RECOVERING OIL FROM AN OUTPUT ZONE. 